Higher Processing of Visual Information Lecture I

--- April 2, 2007 by Mu-ming Poo

1. Overview of the Mammalian Visual System

2. Retinotopic Maps and Cortical Magnification

3. Structure Lateral Geniculate Nucleus (LGN)

4. LGN Receptive Fields

5. Organization of Mammalian Visual Cortices

6. Structure of the Primary Visual Cortex

Part I. Overview of Visual Pathway

Projections of RGCs to the central nervous system

2. Mid-brain projections

3. Central projections

LGN axons project to the primary visual cortex (striate cortex, V1) on the same side

RGC axons project to pretectum, which controls pupillary reflex. RGC axons project to superior colliculus, which controls saccadic eye movement.

1. Optic nerve and optic chiasm

Optic nerve consists of axons of retinal ganglion cells (RGCs).

At the optic chiasm, RGC axons from nasal hemiretina cross and enter the contralateral hemisphere. RGC axons from temporal hemiretina enter the ipsilateral hemisphere.

In cortex, the area devoted to the foveal region of the retina is enlarged relative to the rest of the eye.

Retinotopic map

All structures in the geniculo-cortical system have retinotopic maps.

Cortical magnification

Every point on the map corresponds to a point on the retina.

Cortex Visual field

Fovea: 1 cone => 1 RGC

1 RGC => 2-3 LGN cells

1 LGN cell => 1000 V1 cells

10o periphery: 2 cone => 1 RGC

2 RGCs => 1 LGN cell

1 LGN cell => 100 V1 cells

Visual field

Cortex

LGN

Koniocellular LGN layers – Inter-layers just ventral to each numbered layer - inputs from blue/yellow ganglion cells

Structure of the primate LGN

1

2

3

4

5

6

Magnocellular LGN layers - Layers 1 & 2 - inputs from M-type ganglion cells;

Parvocellular LGN layers - Layers 3,4,5,6 - inputs from P-type ganglion cells;

Each LGN has 6 layers labeled 1 thru 6 from ventromedial to dorsolateral.

50m

Layers 1,3,6 receive contralateral input and Layers 2,3,5 receive ipsilateral input.

Each LGN serves the contralateral visual field. The retinotopic maps from two eyes are in register across the layers. (Axons from RGCs responding to the same visual field innervate LGN cells that are aligned vertically across the layers)

Properties of P, M, and K cells

Only <30% inputs to LGN are from the retina. >70% are feedback inputs, mostly from V1.

Cortical Feedback to LGN: The corticogeniculate pathway

The corticogeniculate axons terminate in all LGN layers in two main patterns (Ichida & Casagrande, 2002)

(1) in individual M or P LGN layers.

(2) Terminate in pairs of M, P, or K layers.

Common functional patternsof corticogeniculate connectivity (Sillito et al, Science 1999):

Feedback to LGN cells along the preferred orientation, enhancing the buildup of orientation selectivity within the geniculo-cortical circuit.

Feedback to LGN cells along the axis of stimulus movement, enhancing responsiveness in anticipation of stimulus movement.

Receptive Fields of RGCs Leaving the retinaReceptive Fields of RGCs Leaving the retina

Color opponency

ON-OFF

Center-surround receptive field

M cells P cells B/Y (K) cells

Each LGN neuron receives input from only a few ganglion cells.

The receptive fields of LGN neurons are concentric with a far stronger surround antagonism than that in RGCs, suggesting the LGN is not a simple relay station.

(Hubel & Wiesel, 1962)

3. LGN receptive fields

Brodmann’s cytoarchitectonic maps of cerebral cortex

Side view of left hemisphere

Inside view of right hemisphere from the midline

The organization of mammalian visual cortices

human

monkey

monkey

V1 has six layers

Anatomy of the primary visual cortex (V1, striate cortex)

Layer 1 is nearly aneuronal, composed predominantly of dendritic and axonal connections.

Layers 2/3 contain excitatory neurons which project to extrastriate cortical regions

Layer 4 is divided into 4A, 4B, 4Ca, and 4Cb. Layers 4Ca and 4Cb are the major recipients of LGN projections.

Layers 5/6 contain excitatory projection neurons which provide feedback to LGN.

Blobs --Cytochrome oxidase (CO) labeling of V1 shows CO rich regions in layer II & III termed "blobs"

Layer 3

Cortical cell types

Excitatory cells: Spiny pyramidal & stellate cells

Inhibitory cells: >40 types

Most LGN cells terminate in layer 4, M in 4Ca, P mostly in 4Cb.

K cells and some P cells terminate in blobs in layers 2 & 3.

Inputs from LGN

Collaterals (branches) of M & P terminate in 6.

Layers II, III & IVB - feedward to extrastriate cortical areas

Layer V - feedback to pons and superior colliculus

Layer VI - feedback to LGN

Output from V1